#include <iostream>
#include <thread>
#include <vector>
#include <chrono>
#include <mutex>
#include <string>

#define SUM_THREASHOLD 32

static std::mutex mtx;

static void sum_krn(int a[], int l, int r, int* res, int thr_i)
{
    int s = 0;

    mtx.lock();
    printf("[sum_krn] thread %d l %d r %d\n", thr_i, l, r);
    mtx.unlock();

    while(l <= r)
        s += a[l++];

    *res = s;
}

static void parallel_sum(int a[], int size, int* res, int threads)
{
    std::vector<std::thread> workers;
    int n_workers;
    int i = 0, l, r, grain;

    if(threads == -1)
        threads = std::thread::hardware_concurrency();

    grain = size / threads;
    n_workers = threads - 1;

    printf("[parallel_sum] size %d threads %d grain %d\n",
            size, threads, grain);

    // skip if the array is too small
    if(size >= SUM_THREASHOLD)
    {
        for(i = 0; i < n_workers; ++i)
        {
            l = grain * i;
            r = grain * (i + 1);

            std::thread thr = std::thread(&sum_krn, a, l, r, res, i);
            workers.push_back(std::move(thr));
        }
    }

    sum_krn(a, grain * i, size - 1, res, i);

    for(i = 0; i < workers.size(); ++i)
        workers[i].join();
}

int main(int argc, char** argv)
{
    int sz;
    int i;
    int sum;
    int threads = -1;
    std::vector<int> a;

    if(argc < 2)
    {
        fprintf(stderr, "usage: prog ARRAY_SIZE [N_THREADS]\n");
        return -1;
    }

    try {
        sz = std::stoi(argv[1]);

        if(argc >= 3)
            threads = std::stoi(argv[2]);
    }
    catch(const std::exception& e)
    {
        fprintf(stderr, "Failed to parse input args: %s\n", e.what());
        return -1;
    }

    a.resize(sz);
    for(i = 0; i < sz; i++)
        a[i] = i;

    auto tm_start = std::chrono::high_resolution_clock::now();

    parallel_sum(&a[0], sz, &sum, threads);

    auto tm_end = std::chrono::high_resolution_clock::now();
    std::chrono::duration<double> tm_diff = tm_end - tm_start;
    double elapsed = tm_diff.count();

    printf("Elapsed time %f sec.\nSum = %d\n", elapsed, sum);
}